linux-kernel - Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup

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Message-ID: <bf71e29f-ae5a-aad0-758d-53293e2105b4@redhat.com>
Date:   Thu, 16 Jan 2020 10:34:20 -0500
From:   Waiman Long <longman@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...ux.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Robert Richter <rrichter@...vell.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2] watchdog: Fix possible soft lockup warning at bootup

On 1/16/20 6:44 AM, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
>
> Added ARM64 and ThunderX folks 
>
>> Waiman Long <longman@...hat.com> writes:
>>> By adding some instrumentation code, it was found that for cpu 14,
>>> watchdog_enable() was called early with a timestamp of 1. That activates
>>> the watchdog time checking logic. It was also found that the monotonic
>>> time measured during the smp_init() phase runs much slower than the
>>> real elapsed time as shown by the below debug printf output:
>>>
>>>   [    1.138522] run_queues, watchdog_timer_fn: now =  170000000
>>>   [   25.519391] run_queues, watchdog_timer_fn: now = 4170000000
>>>
>>> In this particular case, it took about 24.4s of elapsed time for the
>>> clock to advance 4s which is the soft expiration time that is required
>>> to trigger the calling of watchdog_timer_fn(). That clock slowdown
>>> stopped once the smp_init() call was done and the clock time ran at
>>> the same rate as the elapsed time afterward.
> And looking at this with a more awake brain, the root cause is pretty
> obvious.
>
> sched_clock() advances by 24 seconds, but clock MONOTONIC on which the
> watchdog timer is based does not. As the timestamps you printed have 7
> trailing zeros, it's pretty clear that timekeeping is still jiffies
> based at this point and HZ is set to 100.
>
> So while bringing up the non-boot CPUs the boot CPU loses ~2000 timer
> interrupts. That needs to be fixed and not papered over.
>
You are right that the root-causing effort wasn't complete and I took
the easy way out. As I have limited knowledge on how the timer code
work, I was not sure how to start further investigation at that time.
Your insight gives me a hint on where to start now. So I will dig
further in to see what causes this.

Thanks,
Longman